Not to sound mean, but if your FIL does not know the proper code for this installation he is NOT an electrician. Simply knowing how to make it work does now make one an electrician, the codes are half the battle (at least).
The pool guy is no better, he is the last one I would listen to as far as electric codes and practices go.
You DO NOT need an additional box next to the main panel, but you DO need to run conduit fro the main panel to the pool panel. A feeder for a pool panel MUST be in conduit all the way from the main panel to the pool sub-panel.
You need to explain this: "...then run 4 #6 wires to an external load center, then from the load center go to an external pool box"
What is this "pool box" you are referring to?
Also, a 1-1/2hp pump is NOT a "small" pump. All the wiring to pool associated motors and equipment MUST include a #12 (minimum) insulated ground. This means NO NM or UF cable. Everything MUST be in conduit or flexible liquidtite conduit. Motors can have a cord for plug & receptacle connection, but the cords cannot be linger than 3'.
Then there is the issue of bonding. A VERY complex issue with it's own set of rules and codes.
This is NOT an installation to mess around with if you don't know what you are doing. There are TONS of important codes involved with an in-ground pool installation. PLEASE do yourself a favor and enlist the help of a qualified professional electrician. You will be served well by the long term safety of the pool after that.